This invention relates to a hydrocracking process in which the hydrocarbon feedstock has a tendency to form polynuclear aromatic compounds in the hydrocracking reactor and is particularly concerned with a method of removing the heavier polynuclear aromatic compounds from the hydrocracking system in order to prevent their buildup to a level which may cause deactivation of the hydrocracking catalyst and/or fouling of heat exchangers and other process equipment to such an extent that flow through and around such equipment is impeded.
Petroleum refiners often desire to produce products, such as gasoline and turbine fuel, by catalytically hydrocracking high boiling hydrocarbons into product hydrocarbons of lower average molecular weight and boiling point. Hydrocracking is generally accomplished by contacting, in an appropriate reactor vessel, a gas oil or other hydrocarbon feedstock with a suitable hydrocracking catalyst under appropriate conditions, including an elevated temperature and an elevated pressure and the presence of hydrogen, such that a hydrocarbon product is obtained containing a substantial portion of a desired product boiling in a specified range, as for example, a heavy gasoline boiling in the range of 185.degree. to 420.degree. F.
Oftentimes, hydrocracking is performed in conjunction with hydrotreating, usually by a method referred to as "integral operation." In this process, the hydrocarbon feedstock, usually a gas oil containing a substantial proportion of components boiling above a desired end point, as for example, 420.degree. F. in the case of certain gasolines, is introduced into a catalytic hydrotreating zone wherein, in the presence of a suitable catalyst, such as a zeolite- or sieve-free, particulate catalyst comprising a Group VIII metal component and a Group VIB metal component on a porous, inorganic, refractory oxide support most often composed of alumina, and under suitable conditions, including an elevated temperature (e.g., 400.degree. to 1000.degree. F.) and an elevated pressure (e.g., 100 to 5000 p.s.i.g.) and with hydrogen as a reactant, the organonitrogen components and the organosulfur components contained in the feedstock are converted to ammonia and hydrogen sulfide, respectively. Subsequently, the entire effluent removed from the hydrotreating zone is treated in a hydrocracking zone maintained under suitable conditions of elevated temperature, pressure, and hydrogen partial pressure, and containing a suitable hydrocracking catalyst, such that a substantial conversion of high boiling feed components to product components boiling below the desired end point is obtained. Usually, the hydrotreating and hydrocracking zones in integral operation are maintained in separate reactor vessels, but, on occasion, it may be advantageous to employ a single, downflow reactor vessel containing an upper bed of hydrotreating catalyst particles and a lower bed of hydrocracking particles. Examples of integral operation may be found in U.S. Pat. Nos. 3,132,087, 3,159,564, 3,655,551, and 4,040,944, all of which are herein incorporated by reference in their entireties.
In some integral operation refining processes, and especially those designed to produce gasoline from the heavier gas oils, a relatively high proportion of the product hydrocarbons obtained from integral operation will have a boiling point above the desired end point. For example, in the production of a gasoline product boiling in the 185.degree. F. to 420.degree. F. range from a gas oil boiling entirely above about 530.degree. F., it may often be the case that as much as 30 to 60 percent by volume of the products obtained from integral operation boils above 420.degree. F. To convert these high boiling components to hydrocarbon components boiling below 420.degree. F., the petroleum refiner separates the 420.degree. F.+ high boiling components from the other products obtained in integral operation, usually after first removing ammonia by a Water washing operation, a hydrogen-containing recycle gas by high pressure separation, and an H.sub.2 S-containing, C.sub.1 to C.sub.3 low BTU gas by low pressure separation. This 420.degree. F.+ boiling bottom fraction is then subjected to further hydrocracking, either by recycle to the hydrocracking reactor in single stage operation or by introduction into a second hydrocracking zone wherein yet more conversion to the desired 185.degree. to 420.degree. F. product takes place.
The feedstocks to hydrocracking systems, such as the one described above, will normally contain small amounts of compounds known as polycyclic aromatics or polynuclear aromatic compounds commonly referred to as "PNA's." The heavier polynuclear aromatic compounds, i.e., those containing seven or more fused benzene rings, typically have boiling points above about 950.degree. F. and tend to be soluble in hydrocarbon oils only to the extent of several hundred ppmw. Quite frequently additional heavy polynuclear aromatic compounds will form in-situ as the feedstock to the hydrocracking system is first subjected to hydrotreating and then to hydrocracking at elevated temperatures and pressures. High endpoint feedstocks, such as vacuum gas oils, FCC cycle oils and coker gas oils, tend to be especially prone to the formation of heavy polynuclear aromatics during hydrotreating and hydrocracking. Thus, in a hydrocracking system as described above wherein the effluent from a hydrocracking reactor is separated into one or more lower boiling fractions and a higher boiling fraction containing unconverted oil, and this higher boiling fraction is recycled to the hydrocracking reactor, there is a tendency for these heavy polynuclear aromatic compounds to build up in the hydrocracking system and accumulate or "plate out" in cooler portions of the system, particularly heat exchange surfaces, transfer lines, valves and the like, thereby causing plugging problems and reduced heat exchange efficiency. Furthermore, these heavy polycyclic aromatic compounds are known to contribute to catalyst fouling and coking, and therefore their buildup in the hydrocracking system can have deleterious effects on catalyst activity. This is especially true as the hydrocracking temperature is increased. Examples of the types of heavy polynuclear aromatic compounds that can accumulate in a hydrocracking system and cause the above-mentioned problems are shown in U.S. Pat, Nos. 3,619,407 and 5,190,663, the disclosures of which are herein incorporated by reference in their entireties, and include coronene, peropyrene, naphthocoronene, benzocoronene, ovalene, their alkyl substituted derivatives, and the like.
In order to avoid the problems caused by the buildup of polynuclear aromatic compounds in hydrocracking systems wherein unconverted oil is recycled to the hydrocracking reactor, it has been suggested in U.S. Pat. No. 4,447,315, the disclosure of which is herein incorporated by reference in its entirety, that the unconverted oil first be treated with an adsorbent, such as a molecular sieve, silica gel, activated carbon, activated alumina, silica-alumina or clay, to remove these compounds from the oil. Although such adsorbents have a tendency to remove at least some of the heavy polynuclear aromatics from the hydrocracking system, they tend not to be as effective as most refiners would desire, and therefore there is a need for other methods of reducing the buildup of polynuclear aromatic compounds in hydrocracker process streams.